Insulating coating for magnetic sheet material and method for producing the same



United States Patent 3,132,056 INSULATING COATING FOR MAGNETIC SHEETMATERIAL AND METHOD FOR PRGDUCING THE SAME James M. McQuade, Pittsfield,Mass, assignor to General Electric Company, a corporation of New York NoDrawing. Filed May 19, 1961, Ser. No. 111,155 4 Claims. (Cl. 148-315)The present invention relates to coated magnetic sheet material and amethod of making the same. More particularly, the invention concernsrefractory coating material which can be applied to magnetic sheetmaterial such as silicon steel to separate stacked sheets of this typewhen subjected to heat treatment in multi-ply assembly.

The forms of magnetic sheet material with which the invention isconcerned include strip material such as used in wound transformercores, and cut or punched laminations forming stacked transformer coresand other electrical apparatus.

In the process of treating magnetic sheet material of this type to adaptit for use in transformers or other electrical devices, the sheetmaterial is generally wound in the form of a roll, or cut and arrangedin a plurality of stacked sheets, and placed in these forms in an annealing furnace for the purpose of developing the magnetic properties of thesheet material. During the heat treatment'in the furnace with the sheetmaterial either in wound or stacked form, the adjacent surfaces of themagnetic sheet material are in contact with each other overcomparatively large areas, with the result that at the elevatedtemperatures employed for developing the magnetic qualities of thematerial, the adjacent laminations or turns of the material tend tostick together unless some means is provided for separating the surfacesduring the heat treatment.

In the past, magnesium hydroxide (or magnesium oxide to which magnesiumhydroxide converts upon heating) has been used as a coating material forthis purpose. However, when this coating material is subjected to theelevated temperatures of the annealing stage, it reacts with the siliconin the silicon steel sheet and as a resuit there is formed a. glass-likecoating composed of magnesium silicate which is quite hard and veryadherent to the surface of the sheet material. This coating, in fact,has proved so hard that substantial damage has resulted to the punchingdie (or other cutting device) employed for producing the desired shapesof magnetic sheet material, so that the life of the die was undulyshortened. In order to avoid this disadvantage, it has been the practicein the past to remove the glassy coating prior to the punchingoperation, and after the latter process has been completed theinsulating coating was re-applied to serve as an insulation barrier toreduce eddy current loss in the laminated core formed by the sheetmaterial in its use in a transformer, motor or the like. Such removal ofthe glassy coating required a substantial amount of labor and was undulyexpensive in terms of time and cost.

On the other hand, while types of separator coatings other thanmagnesium oxide or hydroxide are known, such other coating compositionshave not proved as effective as magnesium oxide or hydroxide in removingthe sulfur from the silicon steel being processed, a function which isrelatively important in the proper processing of electrical steel.

It is an object of the invention to provide an improved separatorcoating for metallic sheet material which avoids the abovedescribeddisadvantages.

It is a particular object of the invention to provide a separatorcoating for electrical silicon steel sheet material which has thefunction of desulfurizing the silicon steel in the normal annealingprocedures while being 3,132,056 Patented May 5, 1964' readily removableafter such annealing to enable the silicon steel sheet to be punchedinto the desired shape without undue damage to the punching apparatus.

Other objects and advantages will become apparent from the followingdescription and appended claims.

With the above objects in view, the present invention relates to amethod of providing a separator coating on silicon steel sheet materialnormally containing about 25% silicon which comprises applying upon thesilicon steel sheet material a coating of magnesium oxide, applying onthe thus-coated sheet material a coating of aluminum oxide, and heatingthe thus-coated sheet material.

As a result of such a process, a coating is formed on the surface of thesheet material which functions effectively as a separator layer betweenstacked sheets of the silicon steel material while contributing to thepurification of the material during the heat treatment, and yet can bereadily removed so as to avoid damage to the die in subsequent punchingoperations.

In a usual process of producing electrical grade silicon steel of knowntype, strips of silicon steel having about 25% silicon and about 14 milsthick are coated with a layer formed of magnesium hydroxide Mg(OH andafter being wound into a coil, the coated strip is heated at about 1075to 1Q00 C., preferably about 1175 f C., for eight hours for developingthe magnetic properties of the steel by inducing properly orientedcrystal grain growth and removing certain impurities such as sulfur.This heat treatment is often referred to in the art as a grain growthanneal. As a result of such heat treatment, the magnesium hydroxideloses water of hydration and reacts with the silicon in the surfaceregion of the silicon steel strip to form a hard, tightly adherent,glass-like coating of magnesium silicate (Mg SiO on the strip surface.

In accordance with the invention, the formation of such a glass-likecoating is effectively avoided by applying a layer of aluminum oxide onthe magnesium hydroxide coating prior to subjecting the steel strip tothe subsequent heat treatment (grain growth anneal). It has been foundthat such application of aluminum oxide results, after the final anneal,in a powdery coating on the steel surface from which it is easily shakenor rubbed off. Although a coating of aluminum oxide used alone on thesteel surface has little or no desulfurizing effect on the silicon steelduring grain growth anneal, it has been found that the use of aluminumoxide in conjunction with magnesium oxide or hydroxide in accordancewith the invention does not significantly interfere with thedesulfurizing function of the magnesium material.

The following is an example of a process employed in practicing theinvention, it being understood that the procedure described is set forthsimply for purposes of illustration and is not intended to limit thescope of the invention:

Example 1 Silicon steel sheets (24" x 6") containing 3%% silicon andhaving a thickness of 14 mils are dipped into a slurry composed of 7% byweight MgO in water. The thusapplied coatin gof Mg(OI-I) is dried byusing infra-red lamps at about 200 C. to remove excess water. Thisprocess leaves a dry white powder of Mg(OI-I) coating the surface of thesteel sheet. Aluminum oxide (A1 0 in the form of a white powder isapplied to the thus-coated steel by dry dusting (or by dipping the sheetinto a water slurry of the aluminum oxide) using an amount of thealuminum oxide which corresponds at least to a molecular weightproportion relative to the MgO content of the first coating. In thisprocedure, the magnesium oxide deposits in an amount of about .02oz./ft. Hence, at least enough aluminum oxide should 3 be present todeposit in an amount of about .05 oz./ft. and an excess may be usedwithout difficulty. The aluminum oxide coating is then dried withinfra-red lamps. A number of sheets so coated were stacked and clampedbetween steel plates and annealed in a substantially pure argonatmosphere furnace at 1175 C. for eight hours, and then cooled.

As a result of this treatment there was produced a loosely-held whitepowder on the surface of the steel sheets, and the powdery coating thusformed could be readily removed by shaking or rubbing it off.

In comparative tests made to determine the desulfurizing effect of thecombined magnesium hydroxide-aluminum oxide coating as compared to acoating of magnesium hydroxide alone, a number of silicon steel sampleswere prepared and treated in a manner similar to that described inExample I above except that no aluminum oxide coating was applied. Thesesamples as well as those made in accordance with the Example I processwere chemically analyzed to determine their sulfur content. The amountof sulfur in each group prior to anneal was determined to be .019%Analysis of the magnesium hydroxide coated samples showed the presenceof .0045% sulfur, while that. of the combined magnesium hydroxide andaluminum oxide coated samples showed .0055% sulfur. It is evident fromthis test that there was little practical difference in thedesulfurizing capability between the two types of coatings. I

Apparently, only the combination of magnesium hydroxide with aluminumoxide produces the desired dual results of avoiding the formation of atightly adherent, glassy coating while removing sulfur from the steelduring the annealing process. For example, calcium oxide, a refractoryoxide heretofore used as a separator coating, will still produce aglassy silicate coating even in the presence of aluminum oxide.Moreover, while the combination of zirconium oxide with aluminum oxidemay produce a non-glassy separator coating, this combination isunsatisfactory because these materials alone or in combination will notcontribute to desulfurizing of silicon steel.

There is thus provided in accordance with the invention a novel processof providing a separator coating for silicon steel sheet material whichis readily removable to enable the silicon steel sheet to be punchedinto desired shape without undue damage to the punching apparatus, Whilestill permitting effective desulfurizing of the steel during the usualannealing process. The invention may also have particular application tothose silicon steel processes where the separator coating must beremoved prior to treatments applied after the final grain growth annealwhether a punching or cutting operation is involved or not.

While the present invention has been described with reference toparticular embodiments thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the scope of the invention. Therefore, the appendedclaims are intended to cover all such equivalent variations as comewithin the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The method of providing a separator coating on silicon steel sheetmaterial which comprises applying on the sheet material a coating ofmagnesium oxide, applying on the thus-applied coating a coating ofaluminum oxide in an amount of at least about .05 oz./ft. of the surfacecoated, and heating the thus-coated sheet material at elevatedtemperature for developing the magnetic properties thereof.

2. A method of providing a separator coating on silicon steel sheetmaterial which comprises applying on the sheet material a suspension ofmagnesium hydroxide and drying the thus-applied material, applying onthe thus-applied coating of aluminum oxide in an amount of at leastabout .05 oz./ft. of the surface coated, and heating the thus-coatedsheet material at a temperature of 1075 to 1200 C.

3. A method of providing a separator coating on silicon steel sheetmaterial which comprises applying on the sheet material a suspension ofmagnesium hydroxide and drying the thus-applied material, applying onthe thusapplied coating a coating of aluminum oxide in an amount of atleast about .05 oz./ft. of the surface coated, and heating thethus-coated sheet material in multi-ply assembly at elevated temperaturefor developing the magnetic properties thereof with said coatingsseparating the plies.

4. Silicon steel sheet material having thereon a loosely adherentcoating formed in accordance with the process defined in claim 1.

References Cited in the file of this patent UNITED STATES PATENTS2,385,332 Carpenter Sept. 25, 1945 2,394,047 Elsey et al. Feb. 5, 19462,641,556 Robinson June 9, 1953. 2,992,951 Aspden July 18, 19612,992,952 Assmus July 18, 1961

1. THE METHOD OF PROVIDING A SEPARATOR COATING ON SILICON STEEL SHEETMATERIAL WHICH COMPRISES APPLYING ON THE SHEET MATERIAL A COATING OFMAGNESIUM OXIDE, APPLYING ON THE THUS-APPLIED COATING A COATING OFALUMINUM OXIDE IN AN AMOUNT OF AT LEAST ABOUT .05 OZ./FT.2 OF THESURFACE COATED, AND HEATING THE THUS-COATED SHEET MATERIAL AT ELEVATEDTEMPERATURE FOR DEVELOPING THE MAGNETIC PROPERTIES THEREOF.